If i remember correctly the vicious and hard dungeons have monsters which are set - i mean they are always the same and doesn't depend on which monsters are unlocked.For the normal dungeons you get monsters from the pool of unlocked monsters.

All dungeons have a combination of preset monster types as well as some randomly chosen ones. When you "unlock" a monster it becomes a valid possibility to be chosen as one of the random ones. However, if you unlock a dungeon that has the monster as a preset enemy then you will encounter it anyways.

Basically you will always encounter Dragonspawn enemies in Dragon Isles and the Dragonspawn Boss in Cursed Oasis, but Dragonspawn will not start appearing randomly in other dungeons until you "unlock" them.